The insulin receptor of cultured human lymphocytes has been characterized by biosynthetic labeling with radioactive amino acids (methionine and leucine) and sugars (fucose, galactose, glucosamine and mannose). In addition the subunits of the receptor have been labeled by cell surface methodology that reveal proteins (lactoperoxidase/Na125 I) or glycoproteins (galactose oxidase/NaB3H4, periodate/NaB3H4). The receptor has, as shown by all these methods, two major subunits of molecular weights 135,000 and 95,000 and a minor component of 200,000-210,000. The two major subunits are also revealed by affinity cross-linking with 125I-insulin. The 200,000 component appears to be intracellular high mannose glycoprotein. The 210,000 component appears to be a fully glycosylated, but uncleaved, proreceptor by peptide mapping. In addition, further studies suggest that insulin induces the internalization of both major subunits of the receptor and the 95,000 subunit is a transmembrane protein while the 135,000 subunit is exposed only on the extracellular surface of the plasma membrane.